Electrically driven compressors of the above-mentioned type are discussed, for example in US 2009/0254246 A1. As discussed there, an internal combustion engine drives a DC-generator to charge a battery which is connected to the electric drive motor of the compressor. The compressor is connected through an air treatment unit with an air reservoir. An air system controller controls the electric drive motor of the compressor.
If the air pressure in the air system reservoir is less than a set minimum value, the air system controller starts the motor and air compressor to charge the air reservoir, running the motor at higher speeds. As pressure increases in the air reservoir, motor speed is held constant or ramped down. When the pressure in the air reservoir increases to a set maximum value, the air system controller sets the drive motor and the compressor to shut off which stops the air charging operation.
Further, an operation of an air brake system stops the action of the drive motor and the compressor, to save energy when the air brake system is discharging air to stop the vehicle.
Also, the temperature of the compressor can be considered to control the speed of drive motor of the compressor.
Thus, the speed of the drive motor of the compressor is variable based upon the pressure in the air system reservoir and upon certain operation parameters of the vehicle.